This invention relates to an improved method for preparing bismuth strontium calcium copper oxide compounds, and in particular to a method resulting in highly grain aligned thick films of such compounds exhibiting improved superconducting critical current densities.
One of the most critical properties of superconductors from the standpoint of their utility in practical applications is the critical current density, J.sub.c, of the superconductor at the temperature of intended use. Although very high J.sub.c 's, for example about 10.sup.6 A/cm.sup.2 at 77.degree. K., have been demonstrated in epitaxial single crystal films of the new high temperature ceramic superconductors, the best values achieved for polycrystalline forms of the superconductors have been two to four orders of magnitude lower. The difference may be attributed to the anisotropic nature of conduction in the crystal structure, the poor conduction properties at grain boundaries, and the porosity of most sintered bulk materials.
High temperature superconductor thick films (about 5-200 .mu.m thick) have great potential for applications such as those involving chip-to-chip interconnects and passive microwave devices. Although the very high critical current densities demonstrated at 77.degree. K. in epitaxial YBa.sub.2 Cu.sub.3 O.sub.x thin films would be more than sufficient for these kinds of applications, the J.sub.c 's exhibited by polycrystalline high temperature superconductors have been disappointingly low. One approach to this problem is the development of processes which will yield 100% dense material with high current density a-b crystal planes aligned parallel to the direction of current flow, and with minimal grain boundaries which are not parallel to that direction. Such a morphology would take the form of either a single crystal or a textured microstructure with the grains aligned substantially perpendicular to their c-axes.
One method of forming textured microstructures is directional solidification from the melt. S. Jin et al. (Appl. Phys. Lett. 52, 2074 (1988)) disclose a temperature gradient freeze method for making high current density YBa.sub.2 Cu.sub.3 O.sub.x bulk samples. Laser floating zone-melting methods for forming textured fibers and single crystals of Bi--Sr--Ca--Cu--O compounds pounds have been disclosed by D. Gazit et al. (J. Cryst. Growth 91, 318 (1988)), S. Takekawa et al. (J. Cryst. Growth 92, 687 (1989)), and H. D. Brody et al. (J. Cryst. Growth 96, 225 (1989)). However, until the present invention no zone-melting method existed for forming such a textured microstructure in a thick film Bi--Sr--Ca--Cu--O material.
The present application discloses the fabrication of fully dense laser zone-melted Bi--Sr--Ca--Cu--O thick films with high critical current densities or J.sub.c 's, in some cases over 2000 A/cm.sup.2 at 77.degree. K. and over 11,000 A/cm.sup.2 at 60.degree. K. These values are more than three orders of magnitude greater than those of similarly prepared, but sintered films.